So you didnt see the other stand when you walked in? Im guessing if thats the case, they didnt see yours if its a fairly wooded area. At 40 yds it might be hard to pick out if nobody is sitting in it.
I hunt in Northern MN and hunt almost exclusively public land. At first, any little thing people did that I didnt like would p*** me off to no extent, it took me a couple years of realizing that you have to roll with the punches that public land does/does'nt have to offer. I have seen it all, from big jacked up monster trucks plowing their own paths to go cut wood for the winter, to a team of sled dogs behind a roller sled training for races. Sometimes it makes me mad, sometimes I can do nothing but sit there and laugh because its the last thing you would ever think of happening.... On the other hand, deer in that type of environment are a little more used to traffic, noise, etc than an animal on private property that gets very little pressure from other sources.